The cost of a square slab is proportional to thickness

[abhinav]

why proportional to 2 variables mean multiplication and not sum.

Cost = k*t * m* s^2,

and not,
Cost = k*t + m* s^2

as term "directly proportional" is not indicated.

[RonPurewal]

why proportional to 2 variables mean multiplication and not sum.

Cost = k*t * m* s^2,

and not,
Cost = k*t + m* s^2

as term "directly proportional" is not indicated.

"Proportional", absent other qualifiers, is the same thing as "directly proportional". So it's the first one.

Note that, in your second case, the cost is not proportional to either t or s^2!
If you don't see why not, try plugging in (nonzero) values for t and s. Then double t, and see whether the cost will double. It won't.